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1397 the Use of the Oxides Of 6XfDES OE’ PLATINUM IN ORGANIC RGDUCTIONS 1397 [CONTRIBUTION FROM THE CHEMICAL LABORATORYOF THE UNIVERSITY OF ILLINOIS] THE USE OF THE OXIDES OF PLATINUM FOR THE CATALYTIC REDUCTION OF ORGANIC COMPOUNDS. I BY V. VOORHEESWITH ROGERADAMS~ Received April 10, 1922 The use of colloidal platinum and palladium and of platinum and pal- ladium black as catalysts, especially for the reduction of organic compounds, has become very general during the past 25 years. This interest has been due chiefly to the fact that many organic substances can conveniently be prepared only by such methods. The colloidal catalysts are in general somewhat more active and are more readily prepared, but are limited in their use due to the fact that only aqueous, dil. alcoholic or dil. acetic acid solutions may be employed and that difficulty may often arise in iso- lating the products. On the other hand, the use of platinum or palladium black is very convenient, since almost any solvent may be employed and the product isolated merely by filtering the platinum from the reaction solution and evaporation of the solvent. Of platinum and palladium, the former has been much more commonly used and is the one which will be discussed in this communication. The method of preparation of platinum and palladium black, however, has not been given the study which it deserves. The earlier methods for making platinum black are very numerous and do not yield a prodiict which is very active as a catalyst in the reduction of organic compounds. 0. hew*first described the preparation of a satisfactory catalyst for organic re- ductions and his method, except for slight changes by other investigators, is the one which is used almost exclusively at the present time. Loew’s method consists in the treatment of an aqueous chloroplatinic acid solution with formaldehyde and then pre- cipitation of platinum black by the addition of alkali. The product is filtered, dried in vacuo and exposed to air or oxygen before being used. His directions have been changed, chiefly in regard to the temperature of precipitation and the nature of the alkali, first by Willstaetter and Hart;aagain, by Willstaetter and Waldschmidt-Leitz;’ and still again by Feulgen? These changes from the original directions of Loew result in the production of a more effective catalyst and eliminate a few of the experimental difficulties involved, such as that of filtration of the platinum black. Moreover, it is reported that the platinum produced by the modified directions is more uniform in its activity as a catalyst. Even with these improvements, the production of a very active catalyst cannot be relied upon unless the directions are followed with exactness. Drying in a good vacuum for 24 to 48 hours is very necessary if a high activity is to be obtained. This paper is an abstract of a thesis by V. Voorhees presented in partial fulfil- ment of the requirements for the degree of Master of Science in chemistry at the Uai- versity of Illinois. Loew, Ber., 23, 289 (1900). 3 Willstaetter and Hart, ibid., 45, 1472 (1912). Willstaetter and Waldschmidt-Leitz, ibid., 54, 113 (1921). Feulgen, ibid., 54, 360 (1921). 1395 V. VBORHEES WIT” ROGER ADAMS In this laboratory where catalytic reductions have been studied for the past few years, it has often been found that various students, using the same directions for making the catalyst, do not obtain a platinum of like activitj-. The investigation which is described here was carried out in order to determine whether a simpler and more reliable method could be found for the preparation of a highly active platinum catalyst. X brief description of the characteristics of a catalytically active platinum black is given below, since these observations led to the experiments asid results which are now being reported. The presence of oxygen in platinum black is necessary for its activity as a catalyst in the hydrogena- tion of organic compounds. This has been noted by numerous investi- gators6 and tested quantitatively by Willstaetter and Waldschmidt- 1,eit.z.’ In fact, the activity of platinum black seems to depend almost entirely upon the amount of oxygen absorbed. Willstaetter believes that the presence of oxygen is also necessary for catalysis with colloidal platinum, but recent work by Skitas has shown quite conclusively that this is not the case. The form in which absorbed oxygen is present in the platinum black hzis been a question of great interest to chemists. WohlerQhas shown con-- vincingly that when platinum black ahsorbs oxygen there are formed true oxides or hydroxides of platinum. These consist for the most part of platinous oxide or, in the presence of water, platinous hydroxide, and a certain amount of platinic dioxide and its hydrate. From his experi- ments it is indicated that the platinous oxide or hydroxide is the active catalyst, or at least is an intermediate product in the formation of the ac- tive catalyst. lhis conclusion was drawn after a very careful study oi platinum black which had absorbed oxygen and a comparison of its proper- ties with those of the various oxides of platinum. Platinum black prepared by the usual method shows by its activity that it contains about 10 to IS% of platinous oxide and a few per cent. more of the higher oxides of platinum. The reactivity of the platinum ceases immediately as soon as the oxygen present has disappeared. It is well known that in catalytic reductions the oxygen in the platinum is not immediately removed by the hydrogen but reacts only gradually during a considerable period of time. byhen the oxygen in the platinum does become very small in amount the 0 djversikt Finska Vetenskaps-Sac. Forh., 57, 267 (1900); Z. angew. Chem., 39, 21 (1904); Gazz. cFcim. ital., 34, 57 (1904); Ber., 51, 767 (1918); Compt. rend., 158, 409 (1914); Ber., 51, 768 (1918); Akad. Wetenschappen Amsterdam Proc., 29, 424 11917). Ref. 4. In this paper an excellent review is given of the various papers on the mechanism by which the oxygen activates the platinum as a catalyst. See also Ber. 55, 573 (1922). 8 Skita, ibid., 55, 139 (1922). 0 Wohler, ibid., 36,* 3475 (1906). OXIDES OF PL.ITINUM IN ORGANIC REDUCTIONS 1399 reduction slows down or stops altogether. It is merely necessary, however, to agitate the platinum with oxygen or air in order to revivify it. The state of division of a platinum catalyst is also important and re- cent modifications in the preparation of platinum black have had as one of their objects the formation of a more finely divided product. It is most probable that the state of division and the absorption of oxygen go hand in hand. From these properties of platinum black it was concluded that the oxides of platinum, if prepared in a finely divided state, would be as effective catalysts in hydrogenation as any which could possibly be produced. They would be distinctly more active than the best platinum black since the maximum amount of oxygen in the latter as platinum oxides, is at the most only about 20%. Provided the oxide is finely divided, it would probably make little difference what particular oxide of platinum is used, 4nce the hydrogen would reduce the higher oxides to the lower oxide, presumably the active catalyst. If these oxides are usable, even though they may be merely as active as ordinary platinum black, they would have a distinct advanedge over it because of the ease with which many of them may be prepared. For example, certain tedious details such as the careful drying of platinum black could be eliminated. Also the oxides of platinum, since the oxygen is present, could be used when wet, while platinum black precipitated from solution must be thoroughly dried and exposed to the air for the absorption of oxygen before it becomes most active as a catalyst. Experiments fully substantiate the predictions, many of the oxides of platinum are more effective as catalysts than plati- num black made by the usual method. ,111 of the various oxides of platinum described in the literatiire are now being studied as catalysts. Of these, platinous hydroxide formed as de- scribed by the method of Thompsen'o has been made already. This compound is a catalyst and about as active as ordinary platinum black. The necessary conditions of precipitation and the physical characteristics of the product, however, are not ideal and this probably accounts for it5 lack of great activity. It seemed likely that a proper fusion method might be more suitable for the formation of a finely divided oxide which might be easily handled. Yorgensenl' has already described a poorly defined oxide of platinum produced by the fusion of .;odium chloroplatinate with sodium carbonate. Such a method would hardly be e.;pected 117 produce an active catalyst since the temperature o€ such a fusion mistui . must reach 850" or over. Such high temperatures are known to inter- fere often with the activity of a catalyst. Moreover, it has been shown by Wohlerv that both platinou.; oxide and platinic dioxide drconi1:ost. fairly 10 Thompsen, J. prakt Chem , [Z] 15, 294 (1876). 11 Yorgensen, abid., (21 16, 344 (1877). 1400 V. VOORHEES WITH ROGER ADAMS rapidly when held for any length of time at a temperature of 450'. The most appropriate method conceivable for the formation of an oxide of platinum is the one used so often for the oxides of other metals, namely, the fusion of the nitrate.12 In the case of platinum, this may be carried out by fusing chloroplatinic acid and sodium nitrate. Since sodium nitrate fuses at 312', ideal conditions are obtainable.
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